This invention relates to an exhaust gas purification system for an engine for activation of a catalyst at the time of starting the engine and suppressing deterioration of a catalyst due to influence of exhaust gas heat at a high temperature.
In general, since a catalyst used for an exhaust gas purification system of this type is scarcely activated at a low temperature, the catalyst may be disposed at the upstream position of an exhaust passage so as to produce an activation of the catalyst at the time of starting the engine at a low temperature, but if the catalyst is disposed at the upstream position of the exhaust passage, deterioration of the catalyst is caused by the influence of exhaust gas heat at a high temperature of exhaust at the time of operating the engine at a high load, and a problem arises in its durability.
Therefore, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 210116/1982, there is prior art of technique for enhancing purification ratio of exhaust gas and improving durability by arranging two catalysts at a predetermined interval in an exhaust passage, providing a bypass for bypassing the catalyst at the upstream position, further providing a changeover valve at the inlet side of the catalyst at the upstream position, closing the bypass by the changeover valve when exhaust gas is at low temperature to introduce the exhaust gas to the catalyst at the upstream position, closing the inlet of the catalyst at the upstream position by the changeover valve and introducing the exhaust gas to the catalyst at the downstream position through the bypass when the exhaust gas is at relatively high temperature to cool it through the bypass.
According to the above-described prior art, at least two catalysts are required in the exhaust passage. Particularly, in recent V-type and horizontal type engines for controlling exhaust gas purification at respective banks, at least two catalysts must be provided on one bank, and hence there is a problem in which its cost is expensive.